The action of the heart is known to depend on electrical signals within the heart tissue. Occasionally, these electrical signals do not function properly. The maze procedure is a surgical operation for patients with chronic atrial fibrillation that is resistant to medical treatment. In this procedure, incisions are created in the right and left atria to produce an orderly passage of the electrical impulse from the SA node to the atrioventricular node. Blind passageways are also created to suppress reentry cycles. Currently, the lesions may still be created using a traditional cut and sew technique. The scar tissue resulting from the procedure results in a non-conductive lesion.
Ablation of cardiac conduction pathways in the region of tissue where the signals are malfunctioning is now being used to replace the surgical incisions. Ablation is also used therapeutically with other organ tissue, such as the liver, prostate and uterus. Ablation of organic tissue is also used in several surgical procedures, for both diagnosis and therapy.
In one type of procedure, one or more electrodes at the tip of an electrophysiology ablation device allow the physician to measure electrical signals along the surface of the heart (mapping). When necessary, in another type of procedure, the physician can also ablate certain tissues using, typically, radio frequency (RF) energy conducted to one or more ablation electrodes. During tissue ablation, energy is used to create lesions in the tissue for different purposes. High levels of energy are used to cut and remove tissue (electrosurgery). Lower levels of energy are used to cause cell damage but leave the structure intact so that electrical pathways are blocked within the tissue.
A variety of devices, such as catheters, are used to ablate tissue. Typically, such devices include a conductive tip, which serves as one electrode in an electrical circuit. The electrical circuit is completed via a grounding electrode that may also be on the device or may be coupled to the patient. By controlling the level of energy transmitted to the electrode, the surgeon is able to control the amount of heat generated for the purposes described above.
Irrigation of the ablation site cools the electrode. Irrigated ablation is also known to create deeper lesions that are more likely to be transmural. Transmurality is achieved when the full thickness of the target tissue is ablated.
During ablation, irrigation of the ablation site helps to cool the ablation electrodes, thereby reducing overheating in the vicinity of the electrodes. Undesirable consequences of overheating include the excessive coagulation of blood and the unintended destruction of healthy tissue adjacent the ablation site. The efficient cooling of the linear ablation electrode permits longer lesions to be created by permitting higher ablation energy without resulting in excessive electrode heating.
Typically, delivery of irrigation to the site is accomplished using a separate irrigation source which may pump into the ablation device or which may pump directly to the target tissue site. This requires a separate device that may not deliver irrigation as site-specifically as desired.
Furthermore, there is relatively high hydraulic impedance to saline flow at the distal end (towards ablation site) of a typical ablation device. In comparison, the hydraulic impedance to flow is lower at the proximal end (towards user) of the device. This sometimes results in more irrigation fluid being distributed at the proximal end than at the distal end.
Additionally, there may also be difficulties with electrical impedance to saline flow in a typical ablation device. This may be particularly true in a hemostat-type ablation device. In such a device, the target tissue is positioned between the two jaws of the hemostat, both of which carry ablation electrodes. If the tissue is shorter than the length of the hemostat jaws, a saline bridge may form between the hemostat jaws due to the surface tension of the fluid. This saline bridge is a low electrical impedance pathway. Electrical flow may, therefore, occur preferentially towards the bridge and yield unreliable ablation.
Irrigation fluid may also not be evenly distributed along a single electrode because of the impedance factors described above. Uneven distribution of fluid may result in an uneven lesion. In some cases, the tissue may not receive any irrigation in some areas. The electrode may contact the surface of the target tissue in these unirrigated areas, causing sticking or even charring.
Additionally, longer electrodes are sometimes desired to create longer lesions. These electrodes have a larger pressure drop along their length. This results in greater fluid flow from the proximal end than the distal end and thus irrigation is unevenly distributed which may result in sticking of the ablated tissue to the electrode. Currently an electrode of a given length is needed to create a lesion of a given length. If a lesion of a different length is desired, a new electrode must be used.
It would be desirable therefore to provide a means to control and vary irrigation.
It would further be desirable to facilitate control of lesion length.
It would further be desirable to provide a means for evenly irrigating an ablation electrode and concomitant target tissue site.
It would further be desirable to provide a means for evenly irrigating ablation electrodes of variable length.
It would further be desirable to provide a device in which irrigation capabilities and ablation capabilities are integrated.